Typical wet scrubbing systems incorporate sprays, packed towers, or bubbling cascading trays designed to achieve high gas/scrubbing-medium contact area. And they require booster fans to draw the gas flow through the equipment and into contact with the scrubbing media. The necessarily extensive internal structure requires maintenance that usually takes the scrubber off line. Currently available pollutant removal technologies typically address only individual components e.g., to decrease emission of nitrogen oxides (NOx) or to target mercury (Hg). Sole sulfur dioxide (SO2) removal through flue gas desulfurization (FGD) scrubbing achieving only 90-98% removal efficiency may leave substantial reactive sulfur oxides contamination for downstream carbon dioxide (CO2) capture technologies with which to cope.
Especially in the case of emission sources such as municipal refuse (solid waste) incinerators, in which raw gas SO2/NOx concentration ratio is less than approximately 5 to 10, the generic oxidation-absorption chemical process mode is most applicable in providing simultaneous multi-pollutant removal by scrubbing. Therein, using an aqueous solution of an economical oxidant such as hydrogen peroxide (H2O2):
(a) Chemical reactions in an “Initial Stage” for pre-oxidation are:2NO+3H2O2→N2O5+3H2O;Hg(o)+2H2O2→HgO2+2H2O; and
(b) Chemical reactions in a “Final Stage” for removal are:SO2+Ca(OH)2+½O2+H2O→CaSO4.2H2O;N2O5+Ca(OH)2→Ca(NO3)2+H2O;HgO2,gasborne→HgO2,dissolved.
In the case of gas cleaning applications in which the raw gas SO2/NOx concentration ratio is consistently greater than approximately 10, e.g., medium/high sulfur coal-fired boilers, the generic oxidation-reduction chemical process mode may be used alternatively:
(c) Chemical reactions in the “Initial Stage” are:NO+H2O2→NO2+H2O;Hg(o)+2H2O2→HgO2+2H2O; and
(d) Chemical reactions in the “Final Stage” are:SO2+Ca(OH)2+½O2+H2O→CaSO4.2H2O;2NO2+4SO2+4Mg(OH)2→N2+4MgSO4+4H2O;HgO2,gasborne→HgO2,dissolved.
Powerspan, Inc. of Portsmouth, N.H., has, in a 50 MW slip-stream installation at First Energy's Burger Power Station, field demonstrated the alternative oxidation-reduction process mode via its ECO multi-pollutant scrubbing process, which uses ammonia in lieu of the above calcium hydroxide and magnesium hydroxide as chemical reagent. The facility achieves raw gas pre-oxidation by using a proprietary ECO electronic oxidation method.
BOC Gases (British Oxygen) of Murray Hill, N.J., (with many commercial installations including several at oil refinery fluid catalytic cracking units), has commercially applied the above oxidation-absorption process mode by widely field demonstrating its LoTox flue gas pre-oxidation process. In-duct injection of ozone-rich, gaseous oxygen is therein utilized by BOC Gases to oxidize NO/NO2 gases to highly soluble and readily absorbed, low vapor pressure, N2O5 gas.